1. Field of the Invention
The invention generally relates to a control apparatus and method for a vehicle equipped with an automatic transmission. More particularly, the invention relates to a control apparatus and method for a vehicle which executes a neutral control.
2. Description of the Related Art
An automatic transmission mounted in a vehicle is connected to an engine via a torque converter or the like provided in the automatic transmission, and includes a gear change mechanism that provides a plurality of power transmission paths. This automatic transmission is structured so as to automatically switch the power transmission path, i.e., the gear ratio (drive speed), based on the accelerator opening amount and the vehicle speed, for example. A vehicle having an automatic transmission is typically provided with a shift lever which is operated by a driver into any one of various shift positions (such as REVERSE, NEUTRAL, DRIVE). Automatic shift control is performed in these shift positions (usually in the drive position or forward-drive range).
When a vehicle having this type of automatic transmission is stopped with the shift lever in a position corresponding to a forward-drive range, such as DRIVE, driving force from the idling engine is transmitted to the transmission via the torque converter and then to the wheels, resulting in a phenomenon known as “creeping.” Creeping is extremely useful under certain conditions. For example, it helps to keep a vehicle from slipping backwards when starting from a standstill on an incline so that take-off is smooth. When the driver wants a vehicle that is stationary on a flat road to remain in one spot, however, creeping is unnecessary and must be suppressed by operating the brake. That is, the brake is used to suppress the creeping force from the engine and the fuel efficiency of the engine decreases by a corresponding amount.
To improve fuel efficiency, therefore, it has been proposed to put a transmission that is in the forward-drive range into a neutral state including a semi-neutral state that resembles NEUTRAL while still in the forward-drive range and the vehicle is being held stationary by the brake pedal being depressed so as to operate the brake and the accelerator being almost completely closed. A specific friction apply element of the transmission is released at this time (more precisely, this friction apply element is controlled into a slip state).
Many technologies are known which relate to determining the conditions for allowing and prohibiting the execution of this kind of neutral control, executing neutral control, prohibiting neutral control, and shifting from allowing execution to prohibiting execution (i.e., canceling neutral control).
JP(A) 5-157173 discloses a creep control apparatus for a vehicular automatic transmission, which sufficiently improves fuel efficiency and reduces vehicle vibration during creep prevention control (i.e., neutral control), while reliably preventing engine stall following an increase in engine load when the creep prevention control is cancelled. This creep control apparatus for a vehicular automatic transmission is a creep control apparatus which is designed to prevent creep by placing the transmission in a neutral state, even when the automatic transmission is in the forward-drive range, when predetermined conditions are satisfied. This creep apparatus is provided with the following means: i) means for controlling the idle speed of the engine during creep prevention control to be the same as the idle speed when the creep prevention control is not being executed; ii) means for detecting whether, when the predetermined conditions are not satisfied, the reason for them not being satisfied is because of an intended sudden take-off; and iii) means for canceling the creep prevention control by controlling the hydraulic pressure quickly when the reason for the conditions not being satisfied is an intended sudden take-off, and canceling the creep prevention control by controlling the hydraulic pressure slowly when the reason for the conditions not being satisfied is not an intended sudden take-off.
This creep control apparatus for a vehicular automatic transmission controls the idle speed of the engine during creep prevention control to be the same as the idle speed when the creep prevention control is not being executed. As a result, compared with technology which sets the engine speed higher than necessary, it is possible to reliably reduce vehicle vibrations and improve fuel efficiency, which was the primary object of this creep prevention control. Further, the sound of the engine never changes so there is no annoyance to the driver.
As described above, the creep control apparatus disclosed in JP(A) 5-157173 controls the idle speed of the engine during creep prevention control to be the same as the idle speed when the creep prevention control is not being executed. The creep prevention control (i.e., neutral control), however, is executed based on the idle speed when the creep prevention control is not being executed and the actual engine speed. More specifically, if the actual detected engine speed is even slightly higher than the idle speed (which is the same regardless of whether the creep prevention control is being executed or not), the neutral control (i.e., creep prevention control) is cancelled and neutral control is often not executed again while the vehicle is stopped even if the other conditions for starting the neutral control are satisfied.
The engine speed fluctuates in real time in response to the loads from auxiliary devices driven by the rotational force of the engine. If, as the engine speed fluctuates, it becomes even slightly higher than the set idle speed, the neutral control is cancelled, thus shortening the period during which the neutral control is executed and reducing the fuel efficiency improvement effect.
In order to solve this problem, it is possible to simply increase the idle speed (i.e., the engine speed at which neutral control is allowed). If this is done, then even if the load of an auxiliary device changes, causing the engine speed to rise slightly, the neutral control is continued so as to increase the duration of the neutral control for as long as possible so that the fuel efficiency improvement effect can be achieved. Increasing the idle speed in the manner, however, results in the neutral control being continued while the engine speed is increased. As a result, the input clutch remains in the released state (actually the slip state) while the engine speed is high. The higher the engine speed, the greater the heat generated by the slipping of the input clutch, which is the friction apply element placed in a slip state during execution of the neutral control. This generated heat causes problems such as a reduction in the durability of the input clutch.
Further, during execution of the neutral control, the apply pressure of the input clutch is feedback controlled so that a speed ratio of the torque converter (=turbine speed/engine speed) becomes equal to a target speed ratio. At this time, depending on the state of the feedback control, a large difference may remain between the actual speed ratio and the target speed ratio. As a result, when the engine speed is high, the heat generated by the input clutch slipping increases and causes problems such as a reduction in the durability of the input clutch.
In addition, problems such as a reduction in the durability of the input clutch may also occur if the clutch apply pressure of the input clutch remains high for an extended period of time.